Tubular heating elements and magnesia insulation therefor and method of production

ABSTRACT

COMPACTED, GRANULAR, FUSED MAGNESIA USED AS THERMALLYCONDUCTING ELECTRICAL INSULATING IN TUBULAR, ELECTRICAL RESISTANCE ELEMENTS IS SUBSTANTIALLY IMPROVED BOTH IN COMPACTION DENSITY AND ELECTRICAL RESISTIVITY THROUGH THE ADDITION OF 0.1 TO 5.0 PERCENT OF ANY OF A VARIETY OF SUBSTANCES OF LAYER-STRUCTURE CRYSTAL FORM SUCH AS PYROPHYLLITES.

SPECIFIC IMPEDANCE MEGOHM- in.

July 13, 1971 w. VEDDER ETAL TUBULAR HEATING ELEMENTS AND MAGNESIA INSULATION THEREFOR AND METHOD OF PRODUCTION Original Filed Feb. 1, 1968 MgO 2% PYROPHYLLITE I600 I700 I800 TEMPERATURE /N I/E/V TORS.

W/LLEM VEDDER; JOHN 50 UL T2, Jr. *7 'l y Q THE/R ATTO '21 United States Patent 3,592,771 TUBULAR HEATING ELEMENTS AND MAGNESIA INSULATION THEREFOR AND METHOD OF PRODUCTION Willem Vedder, Latham, N.Y., and John Schultz, Jr., Louisville, Ky., assignors to General Electric Company Original application Feb. 1, 1968, Ser. No. 702,474, new Patent No. 3,477,058, dated Nov. 4, 1969. Divided and this application Feb. 24, 1969, Ser. No. 829,818

Int. Cl. H01b 3/02, 3/10 US. Cl. 25263.2 7 Claims ABSTRACT OF THE DISCLOSURE Compacted, granular, fused magnesia used as thermallyconducting electrical insulating in tubular, electrical resistance elements is substantially improved both in compaction density and electrical resistivity through the addition of 0.1 to 5.0 percent of any of a variety of substances of layer-structure crystal form such as pyrophyllites.

This application is a division of Ser. No. 702,474.

The present invention relates generally to tubular, electrical-resistance, heating elements and is more particularly concerned with novel sheathed elements having superior performance characteristics, with a method of making these novel elements, and with a new magnesia-base composition having special utility as thermally-conducting, electrically-insulating, packing material in these elements.

Heating elements of the type comprising an inner, electrically-resistive conductor, a surrounding layer of magnesia, electrical insulation and an outermost protective jacket are widely used in many industrial heating devices as well as in devices such as domestic ranges, dishwashers and water heaters. This type of heating element is much more durable than, for example, exposed resistance wire. Structurally, it usually includes: (1) a coiled resistance wire composed of alloys such as those made up of percent chromium and 80 percent nickel; (2) compacted magnesia powder containing minor amounts of impurities surrounding the resistance coil as an insulator; and (3) an outer protective metal jacket.

Over the long period in which such elements have been in general use, they have been developed and improved to a state of good performance and service life, meeting high safety standards and competing with consistent success with gas and high-frequency current heating devices. At the same time, however, it has long been recognized that a substantial increase in the thermal conductivity of the magnesia insulation employed in these elements would be desirable, and that a sizable increase in electrical resistivity of this material would be even more important. Each of these objectives, however, would have to be realized Without incurring any substantial offsetting disadvantage of cost of production or operation or impairment of efficiency of these elements. To the best of our knowledge, no one heretofore has achieved either of these objectives.

In accordance with this invention based upon our discovery subsequently to be described, tubular heating ele- 3,592,771 Patented July 13, 1971 ments having superior operating characteristics attributable to our achievement of the foregoing objectives can be produced consistently. Moreover, no substantial modification of the principal operations involved in commercial production is required in the manufacture of these elements.

This invention in its method, article and composition aspects is predicated upon our discovery that certain materials in particulate form, when added in amounts as small as 0.1 percent to granular, fused magnesia, increase electrical resistivity in accordance with the foregoing objectives and improve thermal conductance. We have also found that these additive substances, which preferably will be used in amounts of approximately 2.0 percent, but may be used in amounts up to 5 .0 percent, have in common the characteristic that they exist in layer-structure crystal form. Thus non-swelling layer silicates such as pyrophyllites, tales and non-silicate layer-structure materials such as boron nitride are useful as additives in accordance with this invention, except that those having impurities, such as iron or alkali metals, in significant quantities (generally of the order of more than 5.0 percent in the aggregate) are unsuited for this purpose because of the appreciable electronic or ionic electrical conductivity which they would impart to the resulting magnesia mixture. .As disclosed and claimed in the copending application Ser. No. 702,166 filed of even date herewith in the name of Louis Balint and assigned to the assignee hereof and entitled Electrical Insulating Refractory Composition, the foregoing new results and advantages can he obtained through the use of layer-structure clay minerals and, alternatively, through the use of quartz in comparatively small amounts.

Compaction densities approximating those of the compositions of this invention can also be obtained without the use of any of the foregoing additives by surface hydration of the magnesia powder before compacting it in a heating element in the usual manner. This additional discovery or invention is disclosed and claimed in the copending application Ser. No. 809,149 filed Mar. 21, 1969 in the name of Willem Vedder and assigned to the assignee hereof and entitled Tubular Heating Elements and Magnesia Insulation Therefor and Method of Production.

In some way, not totally understood, the additives of the present invention function to increase the electrical resistivity of magnesia powder used as packing in tubular heating elements, having apparently a physical-chemical effect at high temperatures manifesting itself in the form of substantially increased electrical resistivity of the magnesia insulation. This increase is surprising in that the resistivity of the combined materials is substantially greater than that of either material alone. Additionally, these platey powder additives apparently act as lubricants in the compaction operation, thereby functioning to increase the compaction density and the thermal conductivity of the magnesia insulation.

This invention in its composition aspect accordingly in general comprises a uniform powder mixture of granular, fused magnesia and from 0.1 percent to 5.0 percent of an electrically non-conducting additive of layer-structure crystal form. This composition is further characterized in that at about 85 percent of theoretical density, it has a specific impedance of at least 50 megohm-in. at 830 C.

More in detail, the composition of this invention, as indicated above, will preferably contain about 2.0 percent of a non-swelling layer-structure silicate additive such as a pyrophyllite or a talc. Alternatively a layerstructure non-silicate such as boron nitride may be the additive in part or whole. Further, the mixture may include a wide variety of particle sizes both of magnesia and the additive material, the magnesia preferably, however, being a mixture of particle sizes from 40-mesh to below 325-mesh (U.S. Standard Screen sizes). The additive particulate material is suitably of a size or a mixture of sizes within that range. In any case, the additive material preferably will not be of particle size larger than that of the largest magnesia particles of the mixture at the outset of the compaction operation. Also, as indicated above, a mixture of additives may be employed providing they meet the foregoing requirements and providing further that the aggregate amount of the additives is within the range stated above. We have discovered, in fact, that mixtures of pyrophyllite and boron nitride are especially effective additives for the purpose of this invention.

In its article aspect, this invention, generally described, comprises a tubular heating element including a metal sheath, a coaxial coiled resistor in the sheath and a compacted, polycrystalline mass of a magnesia composition of this invention filling the space between the resistor and the sheath. Those skilled in the art will understand that this description of the article applies to the article at the intermediate stage of its production when the composition of this invention has been introduced into the sheath but prior to the time when the article has been thermally cycled to the extent that the identity of the additive may no longer be readily detected.

Finally, in its method aspect, this invention, described broadly, involves the use of the novel composition described above in the production of a tubular heating element including particularly the step of filling the metal sheath with that novel material. Thus, this method centers in a use concept which in itself has novelty independently of the uniqueness of the composition per se.

Referring to the drawings accompanying and forming a part of this specification:

FIG. 1 is an enlarged, side-elevational view of the heating element of this invention, portions being broken away for purposes of illustration; and,

FIG. 2 is a chart bearing curves comparing the specific impedance of typical magnesia insulation with magnesia insulation of this invention, impedance being plotted on a semi-logarithmic scale as a function of temperature.

The heating element of FIG. 1 resembles the heretofore conventional tubular heaters in that it is made up of three principal parts. Thus, a coiled resistance wire 1 is disposed within an outer protective metal jacket 2 and is embedded in and spaced from the jacket by compacted magnesia powder 3 which serves both as a thermal conductor and electrical insulator. In contrast to the prior devices, however, the heating element of FIG. 1 incorporates magnesia powder which has uniquely high electrical resistivity and may also have superior thermal conductivity because of the presence in it of a minor amount of a layer-structure substance such as pyrophyllite.

The FIG. 1 element is suitably fabricated in accordance with the usual practice in the art, the parts being assembled and the element being conditioned at elevated temperature.

Thus, essentially the only significant departure from prior practice in terms of the fabrication operation consists in the use of the new magnesia compositions of this invention, these being substituted for magnesia used in accordance with the prior art practices in order to obtain the special new results and advantages stated above.

Three pyrophyllites and a talc preferred for use in this invention have analyses as follows:

PYROPHYLLITE A Percent SiO 77.0 A1 0 18.0 F6203 Alkalies 0.2 Ign. loss 3.7

Total 99.9

PYROPHYLLITE B Percent A1 0 29.0 SiO 64.0 F6203 0.5 Na O 0.1 K 0 0.2 Ti0 0.4 CaO-l-MgO Trace Ign. loss 5.4

Total 99.6

PYROPHYLLITE C Percent SiO 62.9 A1 0 23.8 CaO 3.0

MgO 0.8 Fe O 0.7 Ign. loss 5.1

Total 96.3

Talc D Percent SiO 51.0 A1 0 7.3 F8203 1.4 MgO 32.5 CnO 0.2 Ign. loss 7.3

Total 99.7

Those skilled in the art will understand that tales and pyrophyllites in their natural forms are hydrosilicates which can be dehydrated upon heating. When used in natural form in preparing the mixtures of this invention, they are dehydrated during the normal annealing or heattreating operation after fabrication of the heating element or possibly during initial operation of the finished unit if such a preliminary heating operation is not involved. Alternatively, the additives can be dehydrated by heating prior to loading the insulation mixture into the heating unit or even prior to the time that these materials are mixed with magnesia. The effects obtained as described above and the special advantages of this invention are realized independently of how and when this dehydration step is carried out.

Those skilled in the art will also recognize that although the plate-like powder additives of this invention can act as compaction aids during forming operations in the course of fabricating heating elements and thus result in improved density of the insulation, the principal benefits described above can be achieved in certain instances without effecting a substantial increase in compaction density of the material.

The following illustrative, but not limiting, examples are offered in the interest of insuring a full and clear understanding of this invention by those skilled in the art and enabling their practice of it without the necessity for experiment to obtain the new results and advantages stated above:

EXAMPLE I To 100 grams of magnesia of minus 40-mesh particle size are added two grams of pyrophyllite A of minus 200- mesh particle size. A portion of the resulting powder mixture is introduced into a nickel-chrome alloy sheath containing a nickel-chrome electrical resistance element, and the powder is compacted therein to a density of 3.05 grams per centimeter, i.e. about 85 percent theoretical density. The resulting element is then annealed at about 1970 F. for from to minutes, at which time it is ready for test. Results of insulation impedance and thermal conductance tests on this element and on an element which difiers only in that the magnesia powder contains no additive are set out as the first and third items in Table I below.

EXAMPLE II Another portion of the mixture prepared in accordance with the description in Example I is mixed with an additional amount of minus ZOO-mesh pyrophyllite A to bring the pyrophyllite content to approximately four percent. On test, a heating element made with this mixture as described in Example I yields insulation impedance and thermal conductance values set out as the fourth item in Table I.

EXAMPLE III To another 100-gram portion of minus 40-mesh magnesia is added 0.10 gram of 325-mesh boron nitride. A heating element test specimen prepared as described above through the use of the resulting mixture yields test results as set forth in the sixth entry in Table I.

EXAMPLE IV Boron nitride of minus 325-mesh is added to magnesia to produce a uniform powder mixture containing three percent boron nitride as described in Example I. A test heating element prepared as described in Example I using this mixture is tested with results stated in the seventh entry in Table I.

EXAMPLE V In still another operation, magnesia and pyrophyllite B and boron nitride powders are mixed together as stated in the foregoing examples to provide a composition containing 97.9 percent MgO particle size of minus 40-mesh pyrophyllite B (minus ZOO-mesh) 2.00 percent and 0.1 percent boron nitride (minus 325-mesh).

Again, on test of a heating unit prepared as described in Example I, it is found that the thermal conductivity of this mixture is superior to that of the standard magnesia and that the insulation resistance and the current leakage resistance of this mixture are far superior to those properties of the standard magnesia. These test results appear as the final entry in Table I.

EXAMPLE VI A magnesia (minus 40-mesh)-0.5 percent pyrophyllite A powder mixture (minus ZOO-mesh) prepared as described in Example I is tested in a heating element test specimen produced as also described in Example I. As indicated by the second entry in Table I, the insulation impedance of this mixture is substantially better than that of the magnesia powder alone and thermal conductance is slightly improved.

EXAMPLE VII Talc D of minus 325-mesh particle size is mixed with magnesia of minus 40-mesh particle size to provide a heating unit magnesia mixture containing about two percent talc. Upon test in a heating unit made as described above, this mixture is found to have insulation impedance greater than standard magnesia alone and a thermal conductance comparing favorable with the compositions of Examples I-IV as shown by the fifth entry of Table I.

EXAMPLE VIII Pyrophyllite B mixed together with magnesia and used to provide a heating unit as described above yields the test results stated in the ninth entry in Table I. Again the materials are of the powder sizes stated in Example I for both the magnesia and the additive.

TABLE I Insulation Thermal impedance conductance (1,700 F.)

(1,625 F. mean), megohms B .t.u.-in.

MgO, no additive 0.45 11.0 MgO plus 0.5% pyrophyllite A 1. 11. 2 MgO plus 2% pyrophyllite A 1. 60 12.0 MgO plus 4% pyrophyllite A. 0. 65 14. 0 MgO plus 2% talc D 0. 60 13. 5 MgO plus 0.1% boron nitride 0. 65 12. 0 MgO plus 3% boron nitride. 0.87 20.0 MgO plus 2% pyrophyllite O 0. 65 12. 9 MgO plus lzapyrophtylllilte B1 1 .0 i 0.61 12. 7 MgO plus 2 0 pyrop y i e p us boron nitride 1. 10 12. 7

As illustrated in FIG. 2, specific impedance of a magnesia insulation containing 2.0 percent of pyrophyllite A compared very favorably with the same magnesia containing no additive over the temperature range from about 1600 to about 1800 F. Thus, at each specific temperature over that range, the specific impedance of the pyrophyllite magnesia mixture additive approached an order of magnitude greater than that of the magnesia containing no such additive and consisting essentially of magnesia powder.

Wherever in this specification and in the appended claims reference is made to percentages or proportions, reference is had to the weight basis rather than the volume basis unless otherwise specifically stated.

By the term non-swelling as used herein and in the appended claims is meant the property of layer silicates like micas of maintaining the distance between layers of the layer structure in the presence of pure water.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims,

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A composition of matter having special utility as a thermally-conducting and electrically-insulating filler for sheathed electric-resistance heaters comprising a uniform mixture of granular fused magnesia and from 0.1 percent to 5.0 percent of an electrically non-conducting additive in particulate form, said additive being of layer structure crystal form and said composition at 85 percent of theoretical density having specific impedance of at least 50 megohm-in. at 830 C. said composition having less than 5.0 percent of iron or alkali impurities.

2. The composition of claim 1 in which the additive is a substance which is a non-swelling layer-structure silicate.

7 3. The composition of claim 1 in which the additive is References Cited boron mmde- UNITED STATES PATENTS 4. The composltion of claim 1 m WhlCh the additive is 3 278 815 10/1966 Boos et al 252 63 2X :1 pyrophyllite.

5. The composition of claim 1 in which the additive is 5 3457O92 7/1969 Term 252 63'5X a talc- JOHN T. GOOLKASIAN, Primary Examiner 6. The composition of claim 1 in which the additive is a mixture of pyrophyllite and boron nitride. KENDELL Assistant Exammer 7. The composition of claim 1 in which the amount of U S c1 X R the additive is about two percent. 10 

